This invention concerns the conditioning of tobacco products, in particular the conditioning of cut lamina and cut mid rib (known as cut stem) by the introduction of steam to a vibratory conveyor wherein the steam passing by way of the perforations includes a component of flow parallel with the supporting surface of the conveyor.
It is well known to subject tobacco products to steam at atmospheric pressure after cutting and before drying in order to expand or puff the tobacco.
This can be achieved by any means which transports the tobacco in a given direction whilst subjecting it to a transverse flow of steam, but with varying effectiveness.
One means is a rotary cylinder with axis slightly inclined to the horizontal to transport the tobacco, enclosing a stationary pipe parallel with the axis carrying a number of steam jets which direct steam onto and at right angles to the moving tobacco.
Another means is a vertical metering tube or column with axial perforated steam tube which directs steam transversely to the tobacco flowing down the tube.
Another means is an enclosed rotary screw conveyor with steam jets arranged in the trough and/or lid which are directed at right angles to the transported tobacco.
Another means is a simple horizontal gauze band conveyor with the upper strand conveying tobacco over an open topped plenum chamber fed with steam which passes through the tobacco at right angles to its motion.
To achieve expansion or puffing of the tobacco it is necessary to heat the tobacco near to the boiling point of the moisture within the tobacco, in order to create the conditions for expansion.
Tobacco is a hygroscopic material and below a critical moisture, which for tobacco is around 40 to 50%, the moisture is "bound" and exerts a vapour pressure below that of free water, e.g. it can be held in capillaries where the vapour pressure is lowered by the concave water surface. Above the critical moisture there is also free "unbound" moisture on the surface of the tobacco or held in voids which exerts the full vapour pressure. (See Elements of Chemical Engineering by Badger and McCabe page 299).
In general the tobacco has to be heated above 100 degrees C. to achieve boiling point. In fact the elevation can be deduced from the equilibration moisture curves. For example for a typical grade of cut stem at a cutting moisture of 33% the elevation is 2 degrees C. and at 27% the elevation is 4 degrees C., whilst at the critical moisture content of 46% the boiling point is that of free water. In practice there is a compensation factor:
When a hygroscopic material, like tobacco, below the critical moisture content is heated by saturated steam it will first absorb the condensation moisture (typically 5% to raise it from 20 to 100 degrees C.) and then continue to absorb moisture at a much slower rate by a reverse wet bulb process, driven by the vapour pressure difference between the steam and the tobacco. But in this case the tobacco rises in temperature above the steam in order to transfer the latent heat. Like the wet bulb an equilibrium temperature difference is established at which the flow of heat from the tobacco to the steam equals the latent heat of condensation. In fact the elevation in temperature is very similar to the elevation in boiling point in the example above.
The heating ability of steam is dependent on it being 100% saturated steam; it is reduced by two factors: superheat and air dilution.
Saturated steam is a vapour and transfers heat by condensation. Very high transfer rates are possible, because as the steam condenses to water it releases a large latent heat and also reduces to 0.06% of the volume, so that further steam flows in to fill the void.
Superheated steam on the other hand behaves as a gas and transfers heat by conduction, with correspondingly low heat transfer rates, only around 1% of the rate by condensation. To compensate high temperature differences must be used.
In addition the heat available from the superheat is very small compared with the latent heat, so again high temperatures must be used.
If steam is diluted by air it lowers the dew point, i.e. the temperature at which the air is saturated. The mixture behaves approximately as a gas until saturation is reached and the heat transfer is effected correspondingly.
At saturation the heat transfer is by condensation again, but the presence of the air introduces a surface film through which the steam must diffuse reducing the heat transfer. The maximum temperature to which the tobacco can be heated becomes effectively the dew point of the mixture. For steam with 10% air the dew point is 2 degrees C. below boiling point and for 20% air 4 degrees C. below boiling point.
The aim of the heating means must be to exclude air and to heat all 100% of the tobacco. Two to three times the theoretical steam flow is used to try to achieve these aims. Even so the effectiveness of the different means varies.
A particularly convenient method of heating the tobacco is by means of a vibrating conveyor tray, with perforations in the tray bottom to provide vertical upward currents of steam flowing transversely to the tobacco flow, convenient because the equipment is simple, compact, does not lose tobacco height and is easily cleaned.
Several examples of this means are known, in which relatively few high pressure steam jets (greater than 1 bar and up to 10 bar) are used to heat the tobacco. There are several disadvantages to this high pressure and small number of jets: viz the effect of "spouting" the tobacco is experienced which interferes with the conveying action making it sensitive to tobacco flow rate and encouraging the entrainment of air; the small number of jets reduces the proportion of tobacco treated; and the tobacco tends to cling to the enclosure extending over the conveyor.
There is a theoretical minimum of steam required to heat the tobacco and to 100 degrees C. dependent on the specific heat of the tobacco and temperature rise. For example 1500 kg/hr of cut stem requires 125 kg/hr of steam to heat it from 20 to 100 degrees C. In practice two to three times this amount is used to compensate for short term variation of flow rate, incomplete utilisation and a surplus to exclude the air.
For example the device described in example 2 of Patent No. GB 2138666A utilises 7 rows of 15 holes each of 0.8 mm in diameter in a tray 0.4 m wide.times.2.0 m long fed with steam at 10 bar square. That is a total steam flow of 220 kg/hr for a tobacco flow rate of 1200 kg/hr, a free area of 0.0066% (area of tray perforated), a mean hole spacing of 94 mm and only 131 holes/m.sup.2.
Another manufacturer uses four widely separated rows of closer pitched holes approximately 20 mm apart. In both cases the jets use high pressure steam above 1 bar, which lifts the tobacco intermittently and interferes with the conveying action.
In practice a compromise has to be found between too much steam which prevents conveying too little which gives poor processing. As a result the system is sensitive to tobacco flow rate.
To prevent "spouting" the energy of each jet must be reduced. For a given steam flow and steam pressure this means more jets of smaller diameter and a point is reached where the diameter is impractically small, so that lower steam pressures must be used.